The present invention relates to a shoulder prosthesis assembly of the type comprising a metaglenoid element formed by a plate provided with a central tubular bushing together with members for anchoring in the glenoid cavity, and a glenoid sphere adapted to be fitted over the metaglenoid element. The glenoid sphere is provided with an axial screw that is free to turn relative to the glenoid sphere. This screw is adapted to be screwed into the bushing of the metaglenoid element. The glenoid sphere has a convex joint surface for co-operating with a concave cup of an associated humeral prosthesis.
The invention also provides a prosthesis forming part of the prosthesis assembly.
A prosthesis of this type is known, for example, from French patent No. 89/13366 (2 652 498) filed in the name of Medinov.
It is known that the rotator cuff is constituted by the set of shoulder muscles which comprise the deltoid, the supraspinatus, the infraspinatus, the infrascapular, and the smaller round. When massive rupture occurs of the cuff, only the deltoid remains, and that is insufficient for enabling the shoulder joint to operate properly, which requires a shoulder prosthesis to be implanted.
Prostheses are known in which the metaglenoid element is fastened to the glenoid cavity of the scapula by a plurality of screws and receives a glenoid sphere having a convex joint surface, which sphere is screwed to the periphery of the metaglenoid element.
Unfortunately, it is found that after being in use for a certain length of time, the glenoid sphere tends to become unscrewed. This causes the glenoid sphere to tilt under the metaglenoid element and destroys the threads . . . . This damage caused to the prosthesis can be sufficient to require a surgeon to remove the entire prosthesis in order to replace it.
To solve this problem, proposals were initially made to cause the facing contacting surfaces of the glenoid sphere and the metaglenoid element to be rough. That technique has turned out to be insufficient.
Proposals were then made to associate the screw of the glenoid sphere with an arrangement of complementary male and female conical bearing surfaces made respectively on the metaglenoid element and the glenoid sphere. The two conical bearing surfaces can then be moved apart or towards each other as a function of the direction in which the screw is driven in order to lock or unlock the system comprising the glenoid sphere and the metaglenoid element. A shoulder prosthesis arranged in that way is described in French patent No. 95/09395 (2 737 107) in the name of Medinov. That prosthesis gives satisfaction.
However, the heads of the screws can pass right through the metaglenoid element at the end of screw tightening if the surgeon cannot feel this end appropriately, since no special retaining means are provided. Under such circumstances, the metaglenoid element is no longer fixed to the glenoid cavity.
Another problem lies in giving the surgeon access to the metaglenoid element, which raises a difficulty with putting the glenoid sphere properly into place on the metaglenoid element. The patient is in a semi-seated position during surgery, such that the surgeon has to approach the metaglenoid element from a cantilevered-out position, with the metaglenoid element being, as it were, at the bottom of a well defined by the tissues and the muscles of the shoulder. Access to the metaglenoid element is narrow and relatively difficult, with poor visibility that makes it impossible to tell whether the glenoid sphere screw is exactly on the axis of the metaglenoid bushing.
It results that the surgeon can position the glenoid sphere incorrectly, so that the axial screw of the glenoid sphere is not exactly on the axis of the corresponding hole in the metaglenoid element. Under such circumstances, screw tightening is started when the thread on the glenoid sphere screw and the tapping in the central hole of the metaglenoid element are not properly engaged. Once the surgeon becomes aware of this faulty insertion, the threads have already become sufficiently damaged to require the entire prosthesis to be replaced. Not only must two prostheses be used, thus considerably increasing equipment cost, but the time required for the operation is doubled.